This invention relates to a formaldehyde-free latex for use as a fiber binder, such as coating for non-woven mats, such as glass mats, or as a saturant or coating for fiber substrates, such as paper. The latex serves to bond the fibers when cured.
Latexes have been widely used as binder or saturants or coatings for non-woven substrates and paper. In paper substrates, the latex binder or saturant contributes to the high strength of the paper. In paper cloth products having a non-woven, randomly-oriented structure, the polymer latex serves as a binder to hold the structure together and provide strength. Examples of paper cloth products include high-strength, high absorbent materials such as disposable items, e.g., consumer and industrial wipes or towels, diapers, surgical packs and gowns, industrial work clothing and feminine hygiene products. Latex binders are also used for durable products, such as carpet and rug backings, apparel interlinings, automotive components and home furnishings, and for civil engineering materials, such as road underlays.
Non-woven substrates can be formed from fiberglass, polyester, rayon, carbon, and the like fibers. These non-woven substrates could be made by wet-laid processes or dry or air-laid processes. In each case, the latex serves to bind the fibers together and provide tensile strength, solvent resistance and/or high temperature strength to the substrate.
The strength and dimensional stability that is required of paper, non-woven and woven fabrics is currently achieved by bonding the fibers in place with a latex that is crosslinkable. The incorporation of functional or reactive monomers into the copolymer backbone allows the copolymer latex to undergo a crosslinking reaction on curing or processing. Many commercially available monomers contain formaldehyde as an impurity or will promote a crosslinking reaction that evolves formaldehyde. Alternatively, the crosslinking can be obtained by post-adding various aminoplasts to the latex polymer. These situations have frequently involved the use of condensates of melamine and formaldehyde or urea and formaldehyde, but, the presence or evolution of formaldehyde in latex binders is not desirable.
A number of patents have tried to eliminate or minimize formaldehyde use or formation in crosslinking. For example, U.S. Pat. No. 4.524,093 to W.E. Devry teaches a fabric coating composition with low formaldehyde evolution in which an aqueous emulsion of alkyl acrylate monomers are copolymerized with acrylonitrile, itaconic acid, and N-methylol acrylamide. The latex binder substantially reduces the evolution of formaldehyde. U.S. Pat. No. 5,021,529 to R.Y. Garrett teaches a formaldehyde-free, self-curing interpolymer. The polymer is derived from an ethylenically unsaturated monomer, a N-alkylol amide of an alpha, beta-ethylenically unsaturated carboxylic acid and itaconic acid. U.S. Pat. Nos. 5,198,492 and 5,278,222 to D.P. Stack teaches a latex binder for cellulose which is especially useful where low formaldehyde emitting applications are involved. The binder is a combination of a non-formaldehyde emitting latex admixed with an aqueous copolymer dispersion of a highly functionalized emulsion copolymer. The functionalized emulsion copolymer taught is a low solids. i.e., 10 to 16% by weight of solids emulsion, made from 10 to 60 phm of an olefinically unsaturated non-ionic organic compound and equal parts of a carboxylic acid and an olefinically unsaturated carboxylic acid hydroxy ester or an olefinically unsaturated amide or mixtures thereof. The need to blend materials to make a useful latex binder puts limits on its usefulness, may limit blendablility and miscibility, and raises low shelf life issues.